The invention relates to a plotting and drafting apparatus having a magnetic guide for maintaining the orientation of a drawing implement on a surface.
For many years it has been known to use a T-square for drawing parallel lines on a drawing, etc. Typically, the drawing is fastened to a drafting board and the T-square has a guide edge which rests upon a straight side edge of the drafting board. The T-square has a straight datum edge extending perpendicularly to the guide edge. The guide edge rides on the straight side edge of the drafting board and is used to guide movement of the T-square in order to maintain the T-square and datum edge in a desired orientation relative to the drafting board. This permits a draftsman to draw parallel lines or to position other instruments. T-square must be used on a board or table having a straight side edge, and furthermore forces applied to the T-square, particularly to an outer end of the datum edge of the T-square, can deflect the T-square from normal alignment. Such forces might be applied, for example, when drawing lines, and therefore, lines drawn adjacent the outer end of the T-square may not be parallel or aligned with lines adjacent an inner end of the T-square. The T-square can therefore lead to inaccurate unparallel lines.
Drafting can also be done using a drafting board with an attached drafting machine. Typically, a drafting machine is fastened to a portion of the drafting board and uses a mechanical arrangement of tracks, endless loops and pulleys, etc. to guide a drawing implement with a datum edge across a surface of the drafting board with great accuracy. Such a machine enables a draftsman to draw parallel lines relatively easily. In addition, drafting machines typically have a drafting head which often has one or more straight edges pivotally mounted thereto. The straight edges can be pivoted to permit the draftsman to draw a line at any angle relative to the horizontal and vertical dimensions of the drafting board. Drafting machines and drafting heads however, require precise machining and can be expensive.
U.S. Pat. No. 3,672,062 describes an apparatus designed to overcome some of the disadvantages with early drafting equipment by using a magnetic roller on a ferrous surface of a drafting board. Magnets in the roller attempt to eliminate relative skidding between two ends of the roller and the board and tend to maintain the roller in alignment. Thus, the roller may be drawn across the surface with minimal slippage against the surface and therefore, tracking of the roller is relatively accurate. It would appear that in order to draw parallel lines on a drawing, it is imperative that the roller not be removed from the drawing. Should the roller be removed, it becomes necessary to re-align the roller on the surface. Thus, it can be difficult to re-establish proper placement of the roller in order to draw parallel lines. Thus, the alignment of the drawing implement depends upon the placement of the magnetic roller on the drafting board.
Besides their uses in drafting, drawing implements such as parallel rulers are used in navigation to transfer bearing and course headings from one portion of a nautical chart to another. Parallel rulers have two parallel rulers each having respective straight edges and being connected to each other by cross-members. The cross-members are pivotal with respect to each ruler and therefore by moving one ruler parallel to the other, the cross-members are rotated and the rulers are further spaced apart or brought closer together while maintaining the respective straight edges parallel at all times.
Usually parallel rulers are used by aligning the straight edge on the first ruler with a particular course heading, holding the first ruler firmly on the chart, and the moving second ruler away from the first ruler. The second ruler is then held firmly on the chart and the first ruler is brought close to the second ruler by the pivotal action of the cross-members. Then, the first ruler is held firmly on the chart and the second ruler is moved away from the first ruler again. This process is referred to as "walking" the rulers across the chart and is used to transfer selected course headings to any portion of the chart.
Parallel rulers are limited in transferring bearings accurately from relatively widely spaced apart positions on the chart due to the limited degree of spacing of the edges of the parallel rulers as limited by the connecting cross-members extending between the rulers. In addition, as the rulers are walked over the chart, slippage can inadvertently occur between the rulers and the chart and introduce an error into the transferred heading. Furthermore, there is a chance of the rulers inadvertently slipping upon taking each "step", and therefore the degree of error in the transferred heading is cumulative. Consequently parallel rulers can result in erroneous headings.
With any of the above-mentioned devices it is sometimes necessary to remove the drawing implement from the drafting board or chart in order to re-position paper or to observe the drawing or chart on which the drawing implement is being used. In addition, it is desirable to maintain alignment of the drawing implement on the surface of the drafting board and it would be of great advantage to be able to draft on a table or flat surface other than a drafting board. Furthermore, it is desirable to reduce the number of moving parts in the alignment mechanism of the drawing implement in order to reduce alignment problems associated with wear, tensioning, slippage, friction, etc. associated with those parts.